This was not true for me. I unchained my thief, pick pocketed him and then sent my unchained thief to hide in the prison. Guex normally runs over and the Blacksmith says "You just missed him". There is a period of about 30 seconds where vendors look for a thief and if they don't find one, they return to their stalls. I'm not saying there may not be some bug related to something else, but I am able to steal his gold and the grove does not turn hostile. Not sure if this is related to Guex (Stands near Wyll). Maybe Guex is dead in your game or something? Or maybe it doesn't happen to me as I almost always unchain my thief when they are stealing so the whole party doesn't get blamed for their actions.
Note that ShadowHearts "Pass without a Trace" ability is broke... instead of adding +10 to stealth, it adds +10 to all skill checks, including pick-pocketing and most dialogue checks. So it is much easier to pickpocket/pass dialogue checks with her nearby and that ability active.
Whats supposed to happen is that anyone you steal from will open a dialog with a party member near by, and you have option to talk you're way out of the situation. Whats happening in the case of this blacksmith is that he has unvoiced voice barks about a theif and "put that back", then eventually turns hostile. He, and everyone else in the same faction or attitude group (I don't know the technical term used for BG3) as him then turn hostile even if you leave before he turns hostile.
